The Use of 237Np in Personal Dosimeters for Fast Neutrons

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Abstract
As the radiator for a damage-track, fast neutron dosimeter, 237Np has advantages over 232Th, the most important being its lower energy threshold. It has not been used in personal dosimeters because of possible hazards from alpha contamination and gamma dose to the wearer. This paper describes the manufacture and properties of two types of Np radiators (as an alloy with Al and a sintered mixture with Au) for which the removable surface activity is less than about 2 dis/min/cm2 foil. Expressions are given for the neutron detection efficiency of detectors having radiators of pure or diluted fissionable material, with or without a protective coating. These show that adequate containment of the activity need not reduce the efficiency of a given amount of Np by more than 50%. A Np-alloy radiator (0.6 mg of Np) producing 270 tracks per rad of fission-spectrum neutrons gives a dose equivalent of 34 mrem in 2000 hr at 5 cm. Circumstances in which thermal neutrons or high energy gamma radiation may interfere with fast neutron measurements are discussed.

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